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Double Down: Traditional Deworming Increases Drug Resistance At Alarming Rate

Traditional deworming methods dictate that all horses on a farm be dewormed with moxidectin or ivermectin at specific intervals, usually every other month or quarterly. Though significantly easier on farm managers, this method increases anthelmintic resistance two to three times more rapidly than other deworming programs, reports The Horse.

Dr. Thomas Geruden, with Zoetis in Belgium, said these results are not surprising as the worm population is continuously pressured for selection of survivable genes in the calendar-based deworming routine.

Geurden and other researchers in Belgium, along with scientists at the Gluck Equine Research Center in Lexington, Ky., studied two farms that utilized the standard deworming protocol on their Belgian draft horses.

The deworming schedule should have corresponded with the timeline of worm egg development, but drug resistance has caused the worm eggs to appear twice as fast as expected.

Fecal egg counts on every horse on the farm were performed every two weeks from April to September for three years. Horses that had more than 250 eggs per gram of manure were given pyrantel embonate, an alternative dewormer. All other horses were treated with standard anthelmintics in the spring and fall, reducing their deworming by half or two-thirds.

The researchers used the fecal egg counts, both before and after treatment, to create a model of worm life cycles that could predict drug resistance over the next 40 years. The team found that the alternative dewormer maintained low egg counts in all horses and slowed drug resistance in worms by 200 to 300 percent.

The scientists also found that the horses considered “high shedders” of worms were almost always under 5 years old. This knowledge might allow horse owners and caretakers to target which horses may need fecal egg counts run more often – and which may need more-frequent treatments.

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The data suggests that fecal samples should be gathered for egg counts between 42 and 56 days after the spring moxidectin/ivermectin treatment, with a second sample taken between days 98 and 112. These numbers will help determine future egg counts and reduce deworming frequency.

The ability to model the rate of anthelmintic resistance using different deworming protocols has been helpful when discussing deworming with both horse owners and vets, the researchers found. A deworming plan specifically tailored to a farm may be more expensive initially, but the less-frequent, targeted deworming will pay off financially in the long run as dewormer resistance slows.

Can A Mechanical Device Improve Equine Lung Function?

Humans who have chronic lung or heart disease often breathe through a device that resists inhalation to increase muscle strength. Called inspiratory muscle training (IMT), this therapeutic technique trains both the diaphragm and the upper airway muscles to become stronger, improving respiratory strength, according to EQUUS magazine.

Dr. Kate Allen of the University of Bristol wanted to see if the technique could be applied to horses. In people, respiratory performance is limited by cardiovascular capacity; in horses, athletic performance is limited by respiratory function. Using IMT to improve the fitness and function of the respiratory muscles allows them to work more efficiently.

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Allen and a study team acclimated 10 Thoroughbreds in steeplechase training to wearing a mask fitted with valves that control the level of resistance to each inhalation. The horses underwent IMT training five days a week for nine weeks. During each session, the horse took 30 resistant breaths, was rested for two minutes, then took 30 more resisted breaths, all while standing still. The resistance was increased every four days, but the trial allowed for an increase or decrease in resistance depending on how the horse tolerated the change. The horses continued normal racing and training schedules during the study, and tests of each horse's respiratory strength were made before and after the IMT.

The scientists found that horses had no issues adapting to IMT training and that it can be used on horses in competition. The next study will look at IMT training and its impacts on physiologic and performance changes in the horses.

Veterinarians: Dewormer Resistance Must Be Addressed To Avert Equine Welfare Disaster

Though veterinarians and equine caretakers around the world have stressed the importance of forgoing the once-standard practice of rotational deworming, a recent study shows that dewormer resistance is still looming. Currently, small redworms and large roundworms are resistant to all available dewormers; no new dewormers are currently in creation.

Members of the British Equine Veterinary Association (BEVA) anthelmintic working group wrote to the Veterinary Record to express their concern over the findings of a small-scale study. Dr. David Rendle and his colleagues state that a “anthelmintic resistance disaster” is looming unless horse owners change horse-keeping ways.

The study found that although there has been an uptick in the number of fecal worm egg counts (FWECs) performed, there has not been a corresponding downward trend in dewormer sales. The BEVA working group gathered information on the number of fecal worm egg counts completed and the sale of dewormers in the U.K. from 2015 to 2018.

Though FWECs increased by 29 percent, the doses of dewormer sold only fell by 2.9 percent over the same period. The sale of these drugs dropped 8 percent between 2015 and 2016, and then rose every year after that.

If the deworming guidelines were being followed correctly, and dewormers would only be given when a FWEC deemed them necessary. There should be at least twice as many FWECs completed as doses of dewormer sold. However, the data shows that there was only one FWEC completed for every 11 doses of dewormer sold.

The authors also point out that moxidectin sales remained high throughout the study period though experts have noted that it should not be used as a routine dewormer in horses.

Small Strongyles Showing Resistance To Last Effective Class Of Dewormer

Martin Nielsen, DVM, Ph.D., Dipl. ACVM, Schlaikjer professor of equine infectious diseases in the Gluck Equine Research Center, presented his research on small strongyles at the University of Kentucky's Equine Research Showcase in February.

He spoke about current data showing resistance among cyathostomins (small strongyles) to the three main groups of anthelmintic (anti-parasitic) drugs.

“The small strongyle parasite, which is the type of parasite that all horses get, is the primary parasite category, the one that we are always deworming for,” Nielsen said.

The first class of anthelmintic drugs introduced and administered to combat small strongyles was the benzimidazoles in the 1960s. Because this class of drugs has been in use the longest, there is wide-spread resistanceto these drugs.

“It is very rare to find these products still working,” Nielsen said.

Pyrantel salts began experiencing resistance in 1996 and Nielsen said it is also likely to find small strongyles resistant to Pyrantel dewormers today.

Macrocyclic lactones (ivermectin and moxidectin) are our “last resort,” with good efficacy, but there have been some questionable early signs of emerging resistance reported, according to Nielsen. In 2020, routine data collected from a farm in Central Kentucky confirmed resistance to macrocyclic lactones among small strongyles.

Nielsen shared data showing that macrocyclic lactones were 100 percent effective against small strongyles in a group of U.S.-bred yearlings, but the same dewormers administered to a group of imported, Irish-bred yearlings failed to remove small strongyle parasites.

“There's no doubt, looking across this data, that we have clear-cut, proven resistance to ivermectin in these imported parasites,” Nielsen said. “The problem is, if we have resistance to everything, what can we deworm with?” he said.

He described that the efficacy of moxidectin was also evaluated against the resistant parasites. Despite this drug being potentially more potent than ivermectin, it did not overcome the resistance and did not provide better efficacy.

Nielsen closed with a final note that ivermectin and moxidectin resistance is occurring in small strongyles. This case was only discovered due to the meticulous testing procedures in place on this particular farm. Without regular testing of deworming efficacy, drug resistant parasites will go undetected and farms will be left without opportunities to intervene before it is too late. Good parasite control starts and ends with testing the dewormers being used and this must be done every year.